Introduction

Forests and trees play a vital role in removing carbon dioxide (CO₂) from the atmosphere, yet they are often excluded from local greenhouse gas (GHG) inventories. This supplement to the global protocol for community-scale greenhouse gas inventories (GPC) addresses that gap by offering standardised but flexible methods for estimating GHG fluxes from forest land and trees on non-forest land. It was developed through pilot tests in over 20 U.S. communities and four international cities to ensure global applicability.

How GHG fluxes on land occur

Land can function as both a source and a sink for GHGs. Carbon is stored in pools such as biomass, deadwood, litter, and soil, and it moves between these pools and the atmosphere due to natural and human activities. Emissions occur through deforestation, disturbances, and degradation; removals occur through growth and restoration. This supplement adopts the gain-loss method, which uses activity data and emission or carbon gain factors. It includes non-CO₂ emissions from fires but excludes emissions from activities like biomass burning for energy.

Overview of steps for estimating land-based fluxes for the GHG inventory

There are four main stages:

• Exploring and selecting data
• Preparing and aligning activity data and factors
• Calculating and annualising fluxes
• Reporting and using results in climate planning.

The process is parallel for forest land and trees on non-forest land. Communities may stratify land by type or ownership to improve the accuracy and utility of their inventories.

Representing land in the inventory

Communities must define their geographic boundary and classify all land into six IPCC land-use categories: Forest land, cropland, grassland, wetlands, settlements, and other land. A land-use change matrix records transitions between these categories and forms the basis of activity data. Categories can be simplified or disaggregated depending on data availability and reporting needs. Spatially explicit, map-based methods are recommended over sample-based approaches.

Establishing the data update cycle for forests and trees on non-forest land

Although inventories must report on a 12-month period, forest and tree data are often only available in multi-year intervals. Annualising data across the cycle (e.g. 2015–2020) allows reporting to align with GPC requirements. This approach helps address irregularities from events like fires and smooths variability in flux estimates.

Selecting and compiling data for the forest and tree inventory

Required data include land-use change, disturbances, tree canopy cover, and forest type. These are used to derive activity data, emission factors (e.g., t C/ha), and carbon gain factors (e.g., t C/ha/year). Communities should use the most locally relevant data available; otherwise, national or IPCC default values may be used. The selection process is iterative and context-dependent.

Calculating GHG fluxes for forest land

Emissions and removals must be calculated for forest land remaining forest land, forest land converted to non-forest land, and non-forest land converted to forest land. The “committed” approach assumes all emissions occur in the year of change, while the “20-year transition” spreads them. Forest disturbances must be separately identified and reported. Stratification by forest type can improve precision.

Calculating GHG fluxes for trees on non-forest land

Fluxes are based on tree canopy gained, lost, or maintained. Tree censuses or remote sensing may be used. If local data are unavailable, IPCC defaults apply (e.g. 2.8 t C/ha/year for nonboreal urban trees). Stratifying by land type (e.g., settlement vs non-settlement) enhances policy relevance.

Inventory reporting

Emissions and removals must be reported separately, and inventories should include both gross and net figures. This improves transparency and supports integration with other sectors.

Incorporating forest and tree GHG fluxes into climate action goals

Communities can set separate or combined targets for emissions and removals. The supplement enables them to include forests and trees in their climate strategies and better track progress toward mitigation goals.